The present invention relates to a burst signal transmission system and, more particularly, to a method and apparatus for the transmission of burst signals between two remote communication terminals.
Ever increasing tendency to the digitizing of communication systems has already resulted in wide use of digital switching systems. To further enhance the efficiency of digital communication system or to promote advanced new services such as integrated speed-data communication, it is a primary requisite to set up thorough digitization down to local subscribers by digitalizing the subscriber loop transmission. Various systems have hitherto been studied and developed for bidirectional digital transmission using an existing pair of subscriber cables. In terms of simplicity in construction, the most hopeful system is a burst signal transmission system of the type which separates a transmission period from a switching network to a subscriber and a transmission period from the subscriber to the switching network from each other and allows bidirectional transmission to occur within a predetermined burst period.
The burst transmission system of such a type will be outlined with reference to FIGS. 1 and 2. In FIG. 1, a master terminal 1 includes a time compressor 12 adapted to compress a signal A to be transmitted into a burst form. An encoder 13 transforms the compressed signal into a line code which is then supplied in a burst mode to a subscriber loop 3 via a time separation switch 14. A subscriber terminal 2, on the other hand, includes a decoder 25 supplied with the input digital signal A through a time separation switch 24. The decoder 25 processes the signal A in burst form into a binary code. A time expander 26 reconstructs the original form of signal from the burst form signal A, or burst A as it will be referred to hereinafter. A synchronization circuit 27 extracts a burst synchronization signal from the burst A to hold burst synchronization. Meanwhile, a signal B to be fed from subscriber terminal 2 to the master terminal 1 is converted into a burst form line code through a time compressor 22 and an encoder 23 and then coupled to the switch 24. After the reception of the burst A, the switch 24 couples the burst form line code B, or burst B as it will be called hereinafter, to the subscriber loop 3. The burst B received by the terminal 1 via the switch 14 is processed by the time expander 16 into the original signal B. A synchronization circuit 17 extracts a burst synchronization signal from the received burst B. Such an exchange of bursts occurs within a predetermined burst repetition period, as shown in FIG. 2. Each burst has to occupy a time period shorter than one half the burst repetition period; the shorter the time period occupied by a burst, the longer the bidirectional transmission distance can be extended.
An example of this conventional burst transmission system is described in "A Digital Subscriber Set" by Jan Meyer at al., IEEE TRANSACTIONS ON COMMUNICATIONS, Vol. COM-27, No. 7, pp. 1096-1103, July issue, 1979 (Reference 1). According to this example, a burst has a synchronization bit SY in its first bit in addition to speech data-samples, a signalling bit and other information bits and is transmitted using bipolar codes. However, the additional synchronization bit causes an increase in the burst length, i.e., the time period occupied by a burst, and, therefore, a decrease in the maximum possible distance of transmission. The situation remains the same even though multi-level codes may be used as the line codes, requiring an increase in the number of symbols in a burst signal.
Another form of burst transmission is proposed in the U.S. Pat. No. 3,974,339 (Reference 2). This proposed system performs the following operation without using such a burst synchronization bit. More clearly, a transmitting end converts a digital signal into line codes having a positive pulse corresponding to a logical "1" of the digital signal and a negative pulse corresponding to a logical "0" of the same, while a receiving end detects a non-zero pulse initially appearing in the burst. A problem has still existed in this type of burst transmission in that, if the difference in number between positive pulses and negative pulses in a burst is significant, the average DC (direct current) level of the burst differs a great deal from the zero level, resulting in a substantial waveform distortion in a subscriber loop having low frequency cutoff characteristics. Such a waveform distortion makes signal regeneration at the transmitting end difficult.